In a hybrid vehicle, the parallel arrangement of an internal combustion engine and an electric machine in the force flow of the drive-train makes it possible, besides a mixed driving mode in which drive torques of the two drive aggregates are superimposed, to also drive under purely electric motor power or under purely internal combustion engine power. In such a parallel hybrid, the alternate connection of the electric machine and the internal combustion engine to a vehicle transmission is effected by means of separating elements and in most cases the internal combustion engine can be drive-connected to the electric machine by way of a separator clutch. The electric machine can be brought into driving connection with the vehicle transmission or the drive output by way of a second, separate clutch, by means of transmission-internal shifting elements or via a torque converter.
With a parallel hybrid drive, during practical driving operation and depending on the hybrid operating strategy applied, the driving mode of the vehicle can change frequently between internal combustion engine powered, electric motor powered and mixed-mode driving. During purely electric driving operation the electric machine is connected to the drive output, while the internal combustion engine is decoupled from the drive-train by a separating element. In such a situation, if the electric machine is to act as an integrated starter-generator and start the internal combustion engine by operating as an electric motor, then a drag start can be carried out in which, by means of the separating element, the internal combustion engine is re-engaged and accelerated to its starting rotational speed.
Alternatively, if there is a separate starting device for the internal combustion engine, an externally assisted start can be carried out in which, independently of the electric machine present for electric motor operation, the internal combustion engine can be started and brought up to a target speed and then, by means of a separating element that has to be closed, connected to the drive-train to provide drive power. A shifting strategy for the transmission stored in at least one control unit of the vehicle, and also a hybrid operation strategy for operating the internal combustion engine and the electric machine, can work in a vehicle of this type in such manner that relatively often a shift command coincides in time with a command to start the engine.
Conventionally, when a gearshift is in progress and a command for a drag start or externally assisted start is recognized, the gearshift is first taken to completion and the system then reacts to the command for the drag start or externally assisted start in order to couple the internal combustion engine to the input shaft of the transmission.
To shorten the time sequence in such situations and to improve the driving comfort and the drive dynamics, methods are already known for example from DE 10 2007 055 828 A1 and DE 10 2007 055 831 A1, in which the internal combustion engine is started even during a gearshift.
Basically, the starting of the internal combustion engine can take place both during a gear ratio change in the transmission and also separately from transmission gearshift processes and, depending on the drive-train configuration and the structure of the transmission, both with and without traction force interruption. However, in doing this it has not previously been taken into account that a transmission gearshift reasonably called for during electric motor driving may possibly no longer even be necessary during hybrid driving, i.e. when the internal combustion engine is drive-effectively coupled into the drive-train, since the electric driving mode and the hybrid driving mode are usually controlled in accordance with different shifting characteristic curves. Because of this it can happen that, for example, an upshift is initiated during electric motor driving operation, then the externally assisted or drag start is carried out, and immediately thereafter, during internal combustion engine or combined driving operation, a downshift takes place. For a driver of the vehicle this can lead to driving phenomena that are not plausibly comprehensible.
Starts of the internal combustion engine and couplings of the internal combustion engine to the transmission, which are directly time-related to transmission gearshifts, can also lead to an overall relatively time-consuming sequence that can affect the driving behavior adversely. This undesired behavior can be intensified in vehicles having automatic transmissions with a relatively large number of gears, since for reasons relating to fuel consumption and/or power, consecutive multiple shifts or nested shifts often take place, as described for example in DE 10 2006 001 899 A1 and in DE 10 2006 026 601 A1.